Aging in laboratory animals is associated with a decline in regular estrous cycling, and capacity to mate, and ability to carry live fetuses to term. Studies correlating aging and the pathways of ovarian biosynthesis of progesterone and estrogen, steroid hormones essential to implantation and pregnancy maintenance, their levels in the circulation, and their modulation at the uterus are lacking. These studies are essential to understanding whether such aspects are implicated in the age-related decline in reproductive function. The current study will examine the pathways of progesterone and estrogen biosynthesis by the ovaries of aged rats at various periods in gestation. The capacity for ovarian progesterone and estrogen formation will be correlated with the serum concentrations of these steroids and with the onset of fetal death. Since hormonal action is modulated by receptors in target tissues the study will also examine steroid hormone receptor content in the uterus of the aged animals. Thus, a composite of ovarian and uterine function will be developed, which will determine whether pregnancy failure in aged rats is preceded by, or coincides with alterations in functions at these sites.